Two-stage compressor



SePt- 16, 1969 L. s. scHlBaYE 3,467,300

TWO- STAGE COMPRES SOR @www Sept. 16, 1969 1 s. sel-"Bava TWO-STAGECOMPRESSOR 4 Sheets-Sheet 2 Filed Jan. 29, 1968 mvEN TOR n LauRtzBenedictus Schlbbye Sept. 16, 1969 L. s. scl-'IIBBYE 3,467,300

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Sept. 16, 1969 L.. a. scHlBBYE TWO-STAGE COMPRESSOR 4 Sheets-Sheet 4Filed Jan. 29. 1968 IIJ@ 38 lNVENTOR Launitz Benedictus Schbby UnitedStates Patent O 3,467,300 TWO-STAGE COMPRESSOR Lauritz BenedictusSchibbye, Saltsjo-Duvnas, Sweden, assignor to Svenska Rotor MaskinerAktiebolag, Nacka, Sweden, a corporation of Sweden Filed .lan 29, 1968,Ser. No. 701,323 `Claims priority, application Great Britain, Feb. 6,1967,

5,592/ 67 Int. Cl. F04c 17/12, 23/00 U.S. Cl. 230-143 2 Claims ABSTRACTF THE DISCLOSURE Two-stage screw compressor having a housing providingtwo separate Working spaces, each composed of two intersecting bores,the axis of each bore of one working space being coaxial -with the axisof a bore of the other working space, pairs of intermeshing male andfemale rotor elements in each working space, a pair of coaxial rotorelements being axially and torsionally fixed to each other and providedwith a thrust bearing in the end plate of the second stage, and the highpressure port of each stage being located at the end plate of thehousing limiting the Working space.

The present invention relates to a two-stage compressor of the meshingscrew rotor type. Such a compressor comprises a housing composed of twobarrel members separated by a partition, and two end plate members. Eachbarrel member provides a working space composed of two intersecting,cylindrical bores with parallel axes and axially limited by highpressure `and low pressure end wall surfaces, provided by the partitionand the adjacent end plate member. At least one bore of one Workingspace is coaxial with a bore of the other working space and separatedtherefrom by the partition. In each of the working spaces intermeshingmale and female rotor elements are located for rotation around axescoinciding with those of the bores. Each rotor element is provided withhelical lands and grooves having an effective wrap angle of less than360 degrees. The lands and grooves of a male rotor element are locatedmainly outside the pitch circle of the rotor element and are providedwith generally convex flanks, whereas the lands and grooves of a femalerotor element are located mainly inside the pitch circle of the rotorelement and are provided with generally concave flanks. A male rotorelement and a female rotor element cooperate with each other and withthe wall surfaces of the working space to form chevron-shapedcompression chambers, each com prising communicating groove portions ofthe cooperating rotor elements. The base ends of the chevron-shapedchambers are located at the high pressure end wall surface of theworking space, whereas the apices of the chevronshaped chambers moveaxially towards the base ends thereof, as the rotors revolve, wherebythe volume of each chevron-shaped chamber decreases. One rotor elementin one working space is connected with an aligned rotor element in otherworking space to form a torsionally and axially rigid rotor unitextending through the partition and provided with radial bearings in theend plate members of the housing and with a thrust bearing in one of theend plate members only, whereas the other axial end of the axially rigidrotor unit must be axially free. The lead angles of the lands andgrooves of the two elements of the rotor unit are directed oppositely toeach other, whereby the apices of the chevron-shaped compressionchambers in the two sections move in opposite directions, so that theaxial forces acting on the two rotor 3,467,300 Patented Sept. 16, 1969elements of the rotor unit are counterdirected and partially balanceeach other. One working space and the two cooperating rotor elementsenclosed therein constitute a first compression stage. This first stageis provided with an inlet port communicating with an inlet channel tothe compressor which port has at least its major portion located at thelow pressure end wall surface ofthe working space or within an lareaadjacent thereto, and an outflow port communicating with an overflowchannel between the two stages which outflow port has at least its majorportion located at the high pressure end Wall surface of the workingspace or within an area adjacent thereto. The second working space andthe two cooperating rotor elements enclosed therein constitute a secondcompression stage. This second stage is provided with an inow portcommunicating with the overow channel which inliow port has at least itsmajor portion located at the low pressure end wall surface of theworking space or within an area adjacent thereto and an outlet portcommunicating with an outlet channel from the compressor which outletport has at least its major portion located at the high pressure endwall surface of the working space or within an area adjacent thereto.

Hitherto it has been common practice to locate the outow port of thelirst compression stage and the outlet port of the second compressionstage at the end walls of the working spaces provided by the partition.Even though this location of the ports is favorable with regard to thepossibilities to keep the clearances between each of the high pressureend wall surfaces of the working spaces and the confronting highpressure ends of the rotor elements within a very small range, which isessential with regard to the internal leakage and thus to the eiciencyof the compressor, this location of the ports means that the radialforces acting upon the rotor elements and trying to deflect them and toseparate them from each other are largest in the area adjacent to thepartition and smallest in the areas adjacent to the end plate members inwhich the bearings are located. For high pressures in the compressor theclearances between the rotor elements at the high pressure ends thereofwill thus owing to the deflection be so large that the internal leakagewithin the compressor will decrease the eiciency more than is tolerable.

The object of the present invention is to reduce the separation of therotor elements without noticeable deterioration of the compressor inother respects. In order to bring about the desired results the highpressure ports of the two stages, i.e. the outflow port of the Iirststage and the outlet port of the second stage, in accordance to theinvention are located adjacent to the end plate members of the housingand the radial bearings for the rigid rotor unit have to be located inthose members in the way already known per se. In this way the largestradial forces will act upon the rotor elements adjacent to the radialbearings thus reducing the deection of the rotor elements andconsequently reducing the separation thereof.

However, this location of the ports is not enough to produce apractically useful compressor as it is also necessary to hold the axialclearances within a very small range. As the rotor unit is not onlyrotatably rigid in order to convey the required power to the differentcompression stages, but also axially rigid in order to provide thelargest possible balancing of the axial forces deriving from thedifferent stages, the rotor unit can be axially journalled only in onesingle thrust bearing. As the most essential axial clearance in thecompressor is the one between the high pressure end wall surface and thehigh pressure ends of the rotor elements of the second compression stagethe thrust bearing for the rigid rotor unit is in accordance to theinvention located in the housing in the end plate member thereof facingthe working space A of the second compression stage. Furthermore inorder to keep the axial clearance between the high pressure end wallsurface and the high pressure ends of the rotor elements of the irstcompression stage at a small value, which clearance is located at theend plate member of the housing facing the working space of the firstcompression stage, it is necessary to hold the thermal deformations ofthe housing and the rotor elements within a very low range. It is,however, not sutiicient that the variations of the axial distancebetween the end plate members of the housing and of the axial distancebetween the remote rotor ends of the rigid rotor unit are the same, butit is also necessary to hold the housing within a very low temperaturerange in order to avoid warping thereof in dependence on the complicatedform of the housing which warping except for increased dimensionsresults in curved end wall surfaces and in disalignment of the rotorbearings. In order to reduce such thermal deformations to acceptablevalues a compressor according to the invention is provided with meansfor injection of liquid into the working spaces of the dilerent stages,whereby the working fluid is effectively cooled and thus the heattransferred to the compressor structure is so limited that thetemperature rise will fall within an acceptable range.

As the female rotor elements are subjected to larger radial forces thanthe cooperating male rotor elements and as the female rotor elementsnormally have a smaller moment of inertia, so that the deliection ofeach female rotor element is considerably larger than the deflection ofthe cooperating male rotor element, a further improvement in acompressor of the actual type can be obtained by composing the rigidrotor unit of two male rotor elements and by making the two female rotorelements separate from each other. In this case the female rotorelements must be journalled also in the partition and provided withseparate thrust bearings. However, as the axial forces acting on each ofthose female rotor elements are much smaller than those acting upon themale rotor elements separate thrust bearings do not mean any problem. Asat least the thrust bearing of the female rotor element of the firstcompression stage is located on a considerable axial distance from thethrust bearing of the rigid male rotor unit the female rotor element ofthe rst compression stage can not be connected with the male rotor unitby means of a synchronizing gear but has to be driven by direct flankcontact between the flanks of the meshing lands and grooves of thecooperating rotor elements. Normally there will be such a iiank contactbetween the cooperating rotor elements also in the second compressionstage.

f The invention will now be described more in detail with reference tothe embodiment thereof shown in the accompanying drawings, in which:

FIG. l is a top view section of a two-stage compressor generally takenthrough the common plane of the rotor axes,

FIG. 2 is a section taken along line II-II in FIG. 4,

FIG. 3 is a section taken along line III-lll in FIG 2, and

PIG. 4 is a section taken along line IV-IV in FIG. 2.

The compressor shown comprises a housing composed of a first barrelmember 10, a second barrel member 12 integral with an end plate member14, a partition 16 located between the barrel members 10, 12 and an endplate member 18 secured to the end of the barrel member remote from thepartition 16.

Each of the two barrel members 10, 12 forms a working space 20 and 22,respectively, generally composed of two cylindrical, intersecting boreswith parallel axes and axially closed by the partition 16 and theadjacent end plate member 14, 18. All four bores have the same diameterand each bore in one barrel member is in alignment with one bore in theother barrel member.

The working space 20 in the barrel member 10 is provided with an inletport 24 having an axial section in the wall of the partition 16 and aradial section inthe portions of -the barrel wall adjacent thereto andcommunicating with an inlet channel 26 provided in the barrel member 10,in which inlet channel a splash plate 28 is located. The working space20 is further provided with an outliow port 30 having an axial sectionin the wall of the end plate member 1'8 and a radial section in theportions of the barrel wall adjacent thereto and communicating with anoverflow channel 32 provided in the barrel mem- ,ber 10, the partition16 and the barrel member 12. The

working space 22 is correspondingly provided with an iniiow port 34having an axial section in the wall of the partition 16 and a radialsection in the portions of the barrel wall adjacent thereto andcommunicating with the overflow channel 32. The working space 22 isfurther provided with an outlet port 36 having an axial section in thewall of the end plate member 14 and a radial section in the portions ofthe barrel wall adjacent thereto and communicating with an outletchannel 38 located in the barrel member 12.

A male rotor element 40 provided with four helical lands and interveninggrooves, mainly located outside the pitch circle of the element andprovided with generally convex flanks, and a female rotor element 42intermeshing therewith and provided with six helical lands andintervening grooves, mainly located inside the pitch circle of theelement and provided with generally concave flanks, are provided withinthe working space 20 and located in coaxial alignment with the boresthereof. A correspondingly shaped pair of intermeshing male and femalerotor elements 44 and 46, respectively, are provided within the workingspace 22.

The male rotor element 40 is provided with a shaft 48 projectingtherefrom and extending through the partition 16, the working space 22and the end plate memfber 14 to form the driving shaft of thecompressor. The shaft is within the working space 22 provided with twoannular interference surfaces 50, 52 separated by a shallow andrelatively wide valley 54. The male rotor element 44 is provided with acentral bore having correspondingly formed interference surfaces and isshrunk on the shaft 48. The rotor element 44 is further nonrotatablyfixed to the shaft 48 by a pin 56. The two male rotor elements 40, 44form thus an axially and torsionally rigid rotor unit. In order to makeit possible to remove the rotor element 44 from the shaft 48 theinterference surface 52 adjacent to the rotor element 40 has a slightlylarger diameter than the surface 50 and the shaft is provided with acentral channel 58 communicating with the valley 54 through a radialchannel 60 for introduction of a high pressure liquid between the shaft48 and the rotor element 44 in order to nullify the interferencetherebetween.

The rigid male rotor unit 40, 44 is mounted in bearings in the end platemembers 14 and 18. The bearing in the end plate member 18 is a radialbearing only and comprises a bushing 62 held in position vby a cover 64vfixed to the end plate member 18 lby means of 'bolts not shown. Thebearing in the end plate member 14 is a combined radial and thrustbearing and comprises a bushing 66 having an end member 68 non-rotatablybut slightly tiltably fixed thereto. The bushing is held in an axialposition determined by the thickness of a washer 70 by means of a ringmember 72. The shaft 48 of the rotor unit 40, 44 is provided with a ringmember 74 nonrotatably and axially fixed thereto which ring member 74cooperates with the end member 68 to form a thrust bearing for axialforces trying to move the rotor unit 40, 44 towards the end plate member18. The bushing 66 is further provided with an end surface facing therotor element 44 and cooperating therewith to form a thrust bearing foraxial forces trying to move the rotor unit 40, 44 towards the end platemember 12. The end plate member 14 is covered by a cap 76 and a sealingdevice 78 is provided at the opening through which the shaft 48projects.

A bore in the partition is located in alignment with the shaft 48projecting therethrough and provided with a sealing device 80 oflabyrinth type cooperating with the shaft 48.

Shims are inserted between the 'barrel member 12 and the partition 16,between the partition 16 and the barrel member 10, and between thebarrel member and the end plate member 18 in order to adjust the axialclearances between the rotor element 44 and the partition 16, betweenthe partition 16 and the rotor element 40, and between the rotor element40 and the end plate memiber 18, respectively.

The two female rotor elements 42, 46 are completely free from eachother. Each of them is mounted correspondingly to the male rotor unit bymeans of a radial bearing in the partition 16 and in a combined radialand thrust bearing in the adjacent end plate member 18, 14.

The partition 16 is provided with an inlet channel 82 for supply ofpressure oil from a source, not shown. From the channel 82 the oil isdistributed to the working spaces 20, 22 through injection openings 84and 86, respectively, to the sealing device 78 and to bearings for thefemale rotor element 42, 46 located in the partition 16. Pressure oil isfurther supplied to the space between the cover 64 and the confrontingend of the male rotor unit 40, 44 for partial balancing of the axialforces acting on the unit. Oil is also supplied to the bearings notspecifically mentioned above. The oil injected into the working spaces20, 22 serves except for as a lubricant between the cooperating flanksof the rotor elements 40, 42, 44, 46 and as a sealing agent for theclearances between the intermeshing rotor elements and between the rotorelements and the walls of the working spaces also as a cooling agent forthe working Ifluid during the compression thereof resulting in a muchlower temperature of the working fluid and thus in a small transfer of-heat to the structure of the compressor which means so low atemperature of the housing and the rotor elements that the thermalexpansion thereof results in negligible deformations only.

The compressor shown acts in the following way. Working fluid issupplied to the inlet channel 26 and passes through the inlet port 24 tothe working space 20, where it enters the grooves of the rotor elements40` and 42. The fluid is then during the rotation of the rotor elements40, 42 transferred to the compression phase of the first stage andcompressed therein and delivered therefrom through the outflow port 30to the overflow channel 32. During the compression, oil is injected intothe working space 20 through the injection opening 84 for cooling,sealing and lubricating purposes. Most of the oil injected flows withthe working fluid to the overflow channel but a certain amount thereofpasses between the rotor elements 40, 42, is intercepted by the splashplate 28 and returned to the working space 20 without mingling with theheat transfer to the non-compressed working fluid. The working fluidpasses through the overflow channel 32 to the inflow port 34 of thesecond working space 22 where it enters the grooves of the rotorelements 44 and 46. The fluid is then during the rotation of the rotorelements 44, 46 transferred to the compression phase of the second stageand further compressed therein and delivered therefrom through theoutlet port 36 to the outlet channel 38 through which it is dischargedfrom the compressor. During the compression, oil is injected into thesecond working space 22 through the injection openings 86 for cooling,sealing and lubricating purposes.

The forces acting between the rotor elements 40, 42,

44, 46 and the Working fluid are directed and distributed in such a waythat the axial forces acting upon the elements 40, -44 of the male rotorunit counteract each other, which together with the axial force actingon the rotor unit by the pressure oil enclosed by the cover 64 reducesthe load on the thrust bearing 68, 74 to a minimum, and that the radialforces acting upon the rotor elements 40, 42, 44, 46 have their maximumvalues close to the end plate members 14, 18 where the radial Ibearingsare 1ocated which means that the deflections of the rotor elements 40,42, 44, 46 are reduced to a minimum.

However, it is a fact that owing to the shape of the rotor profiles theradial forces acting on the female rotor elements 42, 46 are larger thanthose acting on the male rotor elements 40, 44 as well as to torque ofinertia and thus the resistance to bending is smaller for the femalerotor element 42, 46 than for the cooperating male rotor element 40, 44.For this reason the female rotor elements 42, 46 are provided withbearings also in the partition thus still more reducing the deflectionsof' the female rotor elements 42, 46 and consequently reducing theclearances between the rotor elements 40, 42, 44, 46 to a minimum.

The small deflections of the rotor elements and the negligible thermaldeformations thereof and of the housing means that the clearances in thecompressor necessary with respect to the mechanical reliability can bekept smaller than those otherwise necessary which means that theinternal leakage will be further reduced resulting in a still higherefficiency of the compressor.

The invention is thus not limited to the shown embodiment but encloseseverything falling within the scope of the following claims.

What is claimed is:

1. Two-stage compressor of the meshing screw rotor type comprising ahousing composed of two barrel members, a partition and two end platemembers, providing a working space in each of said barrel members, eachworking space being generally composed of intersecting cylindrical boreswith parallel axes and axially limited 'by the partition and theadjacent end plate member, at least one bore of one working space beingin coaxial alignment with a bore of the other working space,intermeshing male and female rotor elements being mounted in the housingfor rotation in each of said working spaces, each rotor element beingprovided with helical lands and intervening grooves having an effectivewrap angle of less than 360, one rotor element in one working spacebeing connected with a coaxially aligned rotor element in the otherworking space to form a torsionally and axially rigid rotor unit, thelands and grooves of each male rotor element being located mainlyoutside the pitch circle of the element and provided with generallyconvex flanks, the lands and grooves of each female rotor element beinglocated mainly inside the pitch circle of the element and provided withgenerally concave flanks, said intermeshing rotor elements cooperatingwith each other and the walls of the working space to formchevron-shaped compression chambers, said compression chambers varyingin volume when the apices thereof move axially as the rotor elementsrevolve, the directions of movement of lsaid apices being opposite toeach other in the two working spaces, one working space constitutingtogether with the rotor elements enclosed therein a first compressionstage and being provided with an inlet port generally located at one endthereof and with an outflow port generally located at the other endthereof, the other working space constituting together with the rotorelements enclosed therein a second compression stage and being providedwith an inflow port generally located at one end thereof and an outletport generally located at the other end thereof, characterized in thatsaid outflow port of the first stage and said outlet port of the secondstage are located at the respective end plate member, that said rigidrotor unit is mounted in a radial bearing in the end plate memberadjacent to said outow port and in a combined radial and thrust bearingin the end plate member adjacent to said outlet port, and that means areprovided for injection of liquid into said working spaces.

2. Compressor as dened in claim 1, in which said rigid rotor unit iscomposed of two male rotor elements and said female rotor elements arecompletely free from each other, each of said female rotor elementsbeing mounted in the partition and in the adjacent end plate member.

8 References Cited UNITED STATES PATENTS DONLEY J. STOCKING, PrimaryExaminer 10 WILBUR J. GOODLIN, Assistant Examiner U.S. C1. X.R.

